Field of the Invention
The invention concerns a method for movement compensation during magnetic resonance imaging, as well as a magnetic resonance apparatus and a non-transitory, computer-readable data storage medium for implementing such a method.
Description of the Prior Art
In a magnetic resonance apparatus, also referred to as a magnetic resonance tomography system, the body to be examined of a person to be examined, in particular a patient, is situated in a scanner of the apparatus, wherein the patient is subjected to a relatively high basic magnetic field, for example 1.5 or 3 or 7 tesla with the use of a main magnet. In addition, gradient fields are activated by the operation of gradient coils. Radio-frequency pulses, for example excitation pulses, are then emitted via a radio-frequency antenna arrangement by suitable antennas, causing the nuclear spins of certain atoms resonantly excited by these radio-frequency pulses to be tilted by a defined flip angle with respect to the magnetic field lines of the basic magnetic field. During the relaxation of the nuclear spins, radio-frequency signals, so-called magnetic-resonance signals are emitted and are received by suitable radio-frequency antennas, and then processed further. Finally, the desired image data can be reconstructed from the raw data acquired in this manner.
Therefore, for a specific measurement, a specific magnetic resonance sequence, also referred to as a pulse sequence, is implemented, which is composed of a series of radio-frequency pulses, for example excitation pulses and refocusing pulses, and gradient pulses to be emitted, which are suitably coordinated thereto, in different gradient axes along different spatial directions. Chronologically coordinated therewith, readout windows are set that predetermine the periods of time in which the induced magnetic resonance signals are acquired.
During magnetic resonance imaging, movement of the examination object can take place. For example, respiratory movement and/or heart movement of the examination object can occur. It is also possible for arbitrary movements of limbs of the examination object to occur. This movement of the examination object can result in an unwanted change to the magnetic resonance image data acquired during the magnetic resonance imaging, leading to motion artifacts in the magnetic resonance image data acquired. The movement can also reduce the quality of the magnetic resonance image data acquired.
Different methods are known for at least partial compensation of such movement of the examination object during magnetic resonance imaging. One known method is retrospective motion correction, which typically compensates the movement of the examination object in the magnetic resonance image data that are reconstructed, following the acquisition of the magnetic resonance raw data.
A further known method for at least partial compensation of the movement of the examination object is prospective motion correction. Prospective motion correction generally includes the detection and correction of a movement of the examination object during the actual acquisition of the magnetic resonance raw data. At the same time, in specific cases, prospective motion correction can take place in real time, or close to real time. Prospective motion correction can also include the determination of motion parameters characterizing a movement made by the examination object following the acquisition of a first part of the magnetic resonance raw data. It is then possible for recording parameters, for example a slice selection and/or a slice orientation for the acquisition of a second part of the magnetic resonance image data to be set using the motion parameters. This enables the acquisition of the magnetic resonance raw data to be adapted to the movement of the examination object during the actual magnetic resonance imaging.
One known possibility for prospective motion correction is the use of image-based navigators during the magnetic resonance raw data acquisition. This can be used particularly advantageously when the magnetic resonance raw data acquisition is performed using a magnetic resonance sequence with a long acquisition duration and/or with lengthy waiting times or dead times during the measurement, during which raw data representing a navigator volume can be acquired. In this case, the acquisition of a navigator volume typically entails the activation of radio-frequency pulses and the reading out of magnetic resonance measurement data in addition to the sequence elements used by the magnetic resonance sequence to record the diagnostic magnetic resonance raw data. The acquired data representing navigator volume are typically used only for prospective motion correction and can be discarded following the conclusion of the magnetic resonance sequence.